Unit 4: Immune System Disorders

1. Autoimmune Diseases

Autoimmunity is a condition in which the body's immune system fails to maintain self-tolerance (the ability to recognize its own components as "self"). This failure leads to an immune response against its own cells and tissues, as if they were foreign invaders.

The immune system creates autoantibodies (antibodies against self-antigens) or autoreactive T-cells, which attack and destroy healthy tissue.

These diseases are classified into two main groups: organ-specific and systemic.

2. Organ-Specific Autoimmune Diseases

In this type, the immune response is directed against a specific antigen found in only one organ or tissue.

a) Hashimoto's Disease (Hashimoto's thyroiditis)

• Target Organ: Thyroid gland.
• Mechanism: Autoantibodies and autoreactive T-cells attack thyroid gland proteins (like thyroglobulin and thyroid peroxidase).
• Result: This leads to the gradual destruction of the thyroid gland, preventing it from producing thyroid hormone. This causes hypothyroidism (underactive thyroid), with symptoms like fatigue, weight gain, and cold intolerance.

b) Myasthenia Gravis

• Target Tissue: Neuromuscular junction (the connection between nerves and muscles).
• Mechanism: Autoantibodies bind to and block or destroy the acetylcholine (ACh) receptors on muscle cells. ACh is the neurotransmitter that signals muscles to contract.
• Result: With receptors blocked, the muscle cannot receive the nerve signal properly. This causes profound muscle weakness and fatigue, often affecting the eyes (drooping eyelids), face, and throat first.

3. Systemic Autoimmune Diseases

In this type, the immune response is directed against multiple, widespread self-antigens found in different organs and tissues throughout the body.

a) Systemic Lupus Erythematosus (SLE)

• Target: Widespread (systemic).
• Mechanism: Characterized by a wide array of autoantibodies, especially anti-nuclear antibodies (ANAs), which target components of the cell nucleus (like DNA and histones).
• Result: These antibodies form immune complexes (clumps of antigen-antibody) that get trapped in small blood vessels in the skin, kidneys, joints, and brain. This triggers widespread inflammation, leading to symptoms like a "butterfly" rash on the face, joint pain, kidney damage, and fatigue.

b) Rheumatoid Arthritis (RA)

• Target: Primarily the synovium (the lining of the joints).
• Mechanism: Autoreactive T-cells and autoantibodies (like rheumatoid factor) attack the joint lining, causing chronic inflammation.
• Result: The inflammation gradually destroys the cartilage and bone within the joint, leading to pain, swelling, stiffness, and eventual joint deformity.

4. Immunodeficiency

Immunodeficiency is a state in which the immune system's ability to fight infectious diseases and cancer is compromised or entirely absent. This makes the person highly susceptible to opportunistic infections (infections caused by pathogens that don't normally cause disease in a healthy person).

• Primary (Congenital) Immunodeficiency: Caused by a genetic defect. The person is born with a missing or impaired part of the immune system.
• Secondary (Acquired) Immunodeficiency: The immune system is damaged *after* birth by an external factor. This is more common.
  • Causes: Malnutrition, certain cancers (e.g., leukemia), immunosuppressive drugs (for transplants or autoimmunity), and infection (e.g., HIV).

5. HIV and AIDS

This is the most well-known example of a secondary immunodeficiency.

a) HIV (Human Immunodeficiency Virus)

• What it is: A retrovirus (it uses reverse transcriptase to insert its RNA genome into the host's DNA).
• Primary Target: HIV specifically infects and destroys CD4+ T-helper cells.

b) AIDS (Acquired Immunodeficiency Syndrome)

• What it is: AIDS is not the virus itself; it is the *syndrome* (a set of symptoms) that develops in the late stages of HIV infection.
• Diagnosis: A person with HIV is diagnosed with AIDS when their CD4+ T-cell count drops below 200 cells/µL (a healthy count is 500-1,500) OR when they develop one or more specific opportunistic infections (e.g., Pneumocystis pneumonia, Kaposi's sarcoma).
• Result: Without a functional immune system, the patient cannot fight off common germs and is at high risk of death from opportunistic infections or certain cancers.